Among various gasification technologies reviewed in a book by Higman and van de Burgt (Gasification, 2003, Elsevier), it becomes clear that new technologies are desired to improve the economics of gasifying low rank coals, particularly coals with high moisture and/or high ash content such as lignite or subbituminous coals.
The entrained flow gasifiers feed coals with particle sizes less than 75 microns either with a dry or slurry feed system. For dry feed methods, the coal moisture has to be less than 5% to prevent coal particles from forming cakes and bridging in the feed system, particularly in the feed system's lock vessel. For the slurry feed system, about 35 wt % water have to be added to make the coal slurry. It is necessary to dry the coal to very low moisture levels before making slurry to avoid more than 40% total water in the slurry. In general, low rank coals contain more than 30% moisture; to dry the coals to less than 5% moisture requires an expensive drying facility with large operating costs, decreasing the overall process efficiency. It is highly desirable to reduce drying duties and operational concerns in processing low rank coals.
U.S. Pat. No. 6,631,698 discloses a circulating fluidized bed reactor which can be used for gasifying low rank coals. However, the apparatus can only be used in an atmospheric application and requires a large foot-print to produce the large amount of synfuel required by a modern chemical or power plant where coal gasifiers are installed.
U.S. Pat. No. 5,560,900 discloses a process based on a pressurized circulating fluidized bed reactor which is also intended to partially oxidize the low rank coals. The concept proposed to process coal is based on more than five decades of experience with low pressure Fluid Catalytic Cracking (FCC) process in the petroleum industry. As a result, the reactor system, as has been disclosed, uses the Riser as a pyrolyzer with the large amount of heat necessary for the pyrolysis reaction is carried by finely divided refractory materials circulating around the reactor loop. How to separate the coal ash generated in the process from the heat carrying materials is one of the issues that are very difficult to deal with and the patent avoids the issue. Further, this reactor has a Mixing zone below the Pyrolysis zone whose diameter is much larger than the Riser diameter to ensure sufficient residence time to heat-up the coal particles fed into the Mixing zone. The minimum gas velocities necessary to entrain circulating solid particles from the Mixing zone causes the gas velocities to be unusually high in the Transport riser, leading to rapid erosion of any internals such as thermowells and erosion of the cyclone wall. Further, as the pyrolysis reaction needs a much longer residence time for completion and prevent tar formation in the product synthesis gas (syngas), the Riser has to be impractically tall in a commercial process for the reactor proposed in the patent. Also, this process does not teach how to distribute the gas (steam and air or oxygen) appropriately across the cross-section of a commercial size gasifier.
Moving bed gasifiers have been used to gasify low rank coals for more than 100 years. The Lurgi gasifier, in particular, has been widely used to produce syngas for chemical synthesis. However, the moving bed gasifier requires lump coal as feed and cannot utilize the coal fines which are in abundance that often has few users nearby. Another drawback of this technology is that much of the coal is converted into tar rather than useful syngas.
Moreover, all these gasifiers have complicated internals. The moving bed gasifiers have an elaborate rotating grid system and a stirring mechanism as internals for caking coals. The fluidized bed gasifiers have various types of complicated internal gasification agent distributors which are made of exotic alloys to tolerate the gasifier operating temperatures up to 1100° C. In spite of considerable efforts in designing the distribution grids and selecting expensive high temperature alloy materials, failures of these grids still occur which is commercially unacceptable. In the case of entrained flow gasifiers, the most problematic internal is the coal burner which is one of the most maintenance intensive items in the process.
This invention provides an improved apparatus for a wide range of circulating fluid bed applications including gasification of low rank coals such as lignite and subbituminous coals.